Effects study of injection strategies on hydrogen-air formation and performance of hydrogen direct injection internal combustion engine

Based on the dual challenges of the global energy crisis and environmental pollution, hydrogen has been recognized as an ideal alternative internal combustion engine (ICE) fuel. To improve the combustion efficiency of hydrogen direct injection ICE, we numerically analyzed the effects of different injection parameters, including injection timing, injection pressure, and dual injection, on the formation of a hydrogen-air mixture using the CONVERGE software from the perspective of mass transfer and flow state. It was determined that it is enough to set the injection timing to −88° after top dead center (ATDC) for both uniform mixture and desirable indicated thermal efficiency (ITE). However, when the injection timing is set to −43° ATDC, an acceptable ITE and effective combustion can be achieved by employing the “jet-room coordination” effect of the ω chamber. Injection pressure has a minimal effect on mixture formation and combustion. In contrast, the timing and mass fraction of secondary injection have a significant influence on tumble strength, which is a key factor for the mixture improvement.     

Effect of fuel injection timing and injection pressure on performance in a hydrogen direct injection engine

The in-cylinder hydrogen fuel injection method (diesel engine) induces air during the intake stroke and injects hydrogen gas directly into the cylinder during the compression stroke. Fundamentally, because hydrogen gas does not exist in the intake pipe, backfire, which is the most significant challenge to increasing the torque of the hydrogen port fuel injection engine, does not occur. In this study, using the gasoline fuel injector of a gasoline direct-injection engine for passenger vehicles, hydrogen fuel was injected at high pressures of 5 MPa and 7 MPa into the cylinder, and the effects of the fuel injection timing, including the injection pressure on the output performance and efficiency of the engine, were investigated. Strategies for maximizing engine output performance were analyzed.The fuel injection timing was retarded from before top dead center (BTDC) 350 crank angle degrees (CAD) toward top dead center (TDC). The minimum increase in the best torque ignition timing improved, and the efficiency and excess air ratio increased, resulting in an increase in torque and decrease in NO_x emissions. However, the retardation of the fuel injection timing is limited by an increase in the in-cylinder pressure. By increasing the fuel injection pressure, the torque performance can be improved by further retarding the fuel injection timing or increasing the fuel injection period. The maximum torque of 142.7 Nm is achieved when burning under rich conditions at the stoichiometric air-fuel ratio.     

Numerical study on effects of hydrogen direct injection on hydrogen mixture distribution,combustion and emissions of a gasoline/hydrogen SI engine under lean burn condition

A numerical study on effects of hydrogen direct injection on hydrogen mixture distribution, combustion and emissions was presented for a gasoline/hydrogen SI engine. Under lean burn conditions, five different direct hydrogen injection timings were applied at low speeds and low loads on SI engines with direct hydrogen injection (HDI) and gasoline port injection. The results were showed as following: firstly, with the increase of hydrogen direct injection timing, the hydrogen concentration near the sparking plug first increases and then decreases, reaching the highest when hydrogen direct injection timing is 120°CA BTDC: Secondly, hydrogen can speed up the combustion rate. The main factor affecting the combustion rate and efficiency is the hydrogen concentration near the sparking plug: Thirdly, in comparing with gasoline, the NO_X emissions with hydrogen addition increase by an average of 115%. For different hydrogen direct injection timings, the NO_X emissions of 120°CA BTDC is the highest, which is 29.9% higher than the 75°CA BTDC. The hydrogen addition make the NO_X emissions increase in two ways. On the one hand, the average temperature with hydrogen addition is higher. On the other hand, the temperature with hydrogen addition is not homogeneous, which makes the peak of temperature much higher. In a word, the main factor of NO_X emissions is the size of high temperature zone in the cylinder: Finally, because the combustion is more complete, in comparing with gasoline, hydrogen addition can reduce the CO and HC emissions by 32.2% and 80.4% respectively. Since a more homogeneous hydrogen mixture distribution can influence a lager zone in the cylinder and reduce the wall quenching distance, these emissions decrease with the increase of hydrogen direct injection timing. The CO and HC emissions of 135°CA BTDC decrease by 41.5% and 71.4%, respectively, compared to 75°CA BTDC.     

Effects of direct water injection on engine performance in a hydrogen (H2)-fueled engine at varied amounts of injected water and water injection timing

In this paper, the effects of direct water injection (WI) on characteristics of combustion and emission for a hydrogen (H₂)-fueled spark ignition (SI) engine were experimentally investigated. The experiments conducted under different amounts of water injection (AWI) and varied water injection timing (WIT). The experimental results showed that in-cylinder pressure decreased, indicated thermal efficiency (ITE) increased, and the flame development (CA0-10) and propagation (CA10-90) periods prolonged when AWI raised. When AIW grew to 4.5 mg/cycle, Nitrogen oxides (NOx) expelled from the original engine decreased by 53.7% when excess air ratio (λ) was 1.15. Early WIT had positive effects on the reduction of NOx emissions. When WIT retarded, in-cylinder pressure increased, ITE decreased and CA0-10 and CA10-90 shortened, NOx emissions rapidly increased.     

Study on combustion and emission characteristics of a n-butanol engine with hydrogen direct injection under lean burn conditions

The n-butanol fuel, as a renewable and clean biofuel, could ease the energy crisis and decrease the harmful emissions. As another clean and renewable energy, hydrogen properly offset the high HC emissions and the insufficient of dynamic property of pure n-butanol fuel in SI engines, because of the high diffusion coefficient, high adiabatic flame velocity and low heat value. Hydrogen direct injection not only avoids backfire and lower intake efficiency but also promotes to form in-cylinder stratified mixture, which is helpful to enhance combustion and reduce emissions. This experimental study focused on the combustion and emissions characteristics of a hydrogen direct injection stratified n-butanol engine. Three different hydrogen addition fractions (0%, 2.5%, 5%) were used under five different spark timing (10° ,15° ,20° ,25° ,30° CA BTDC). Engine speed and excess air ratio stabled at 1500 rpm and 1.2 respectively. The direct injection timing of the hydrogen was optimized to form a beter stratified mixture. The obtained results demonstrated that brake power and brake thermal efficiency are increased by addition hydrogen directly injected. The BSFC is decreased with the addition of hydrogen. The peak cylinder pressure and the instantaneous heat release rate raises with the increase of the hydrogen addition fraction. In addition, the HC and CO emissions drop while the NOx emissions sharply rise with the addition of hydrogen. As a whole, with hydrogen direct injection, the power and fuel economy performance of n-butanol engine are markedly improved, harmful emissions are partly decreased.     

Effect of mixture strength and injection timing on combustion characteristics of a direct injection hydrogen-fueled engine

An experimental investigation was performed to characterize the hydrogen combustion in a spark-ignition direct-injection engine. It was focused on the effects of mixture strength and injection timing on the characteristics of hydrogen combustion. For this purpose, the practical tests were carried out on an experimental test rig. It is originally designed for optimization of the direct-injection natural-gas engine. The experimental test-rig results comprised the traces for the in-cylinder pressure, mass fraction burned, and heat release rate under the different operation conditions. The results obtained show that the richer mixture condition produced higher pressure trends at all tested points. Besides that, it exhibited a faster rate of increase in combustion rate due to the increase of flame speed. However, the combustion characteristics deteriorated due to the lack of mixture stratification with earlier injection timing. It is concluded that direct-injection timing is essential to achieve better combustion performance. Moreover, retarding the spark ignition timing is also crucial to avoid abnormal combustion in the case of a richer mixture and early start on injection.     

An experimental study of a direct injection compression ignition hydrogen engine

This paper describes the development of an experimental setup for the testing of a diesel engine in the direct injection hydrogen-fuelled mode. Test results showed that the use of hydrogen direct injection in a diesel engine gave a higher power to weight ratio when compared to conventional diesel-fuelled operation, with the peak power being approximately 14% higher. The use of inlet air heating was required for the hydrogen-fuelled engine to ensure satisfactory combustion, and a large increase in the peak in-cylinder gas pressure was observed. A significant efficiency advantage was found when using hydrogen as opposed to diesel fuel, with the hydrogen-fuelled engine achieving a fuel efficiency of approximately 43% compared to 28% in the conventional, diesel-fuelled mode. A reduction in nitrogen oxides emission formation of approximately 20% was further observed.     

Effect of hydrogen direct injection on natural gas/hydrogen engine performance under high compression-ratio conditions

In traffic transportation, the use of low-carbon fuels is the key to being carbon-neutral. Hydrogen-enhanced natural gas gets more and more attention, but practical engines fueled with it often suffer from low engine power output. In this study, the inner mechanism of hydrogen direct injection on methane combustion was optically studied based on a dual-fuel supply system. Simultaneous pressure acquisition and high-speed direct photography were used to analyze engine performance and flame characteristics. The results show that lean combustion can improve methane engine's thermal efficiency, but is limited by cyclic variations under high excess air coefficient conditions. Hydrogen addition mainly acts as an ignition promoter for methane lean combustion, as a result, the lean combustion limit and thermal efficiency can be improved. As for hydrogen injection timing, late injection can increase the in-cylinder turbulence intensity but also the inhomogeneity, so a suitable injection timing is needed for improving the engine's performance. Besides, late hydrogen injection is more effective under lean conditions because of the reduced mixture inhomogeneity. The current study shall give some insights into the controlling strategies for natural gas/hydrogen engines.     

Investigation of the gas injection rate shape on combustion,knock and emissions behavior of a rotary engine with hydrogen direct-injection enrichment

The application of hydrogen direct-injection enrichment improves the performance of gasoline Wankel rotary engine, and the hydrogen injection strategy has a significant impact on combustion, knock, and emissions. The Z160F Wankel rotary engine was used as the investigated compact engine, and the simulation model was developed using CONVERGE software. The combustion, knock and emissions characteristics of the engine were studied with the different mass flow of hydrogen injection, i.e., the trapezoid, wedge, slope, triangle and rectangle type of gas injection rate shape. In the numerical simulations, the in-cylinder pressure oscillations were monitored using monitoring points, and the knock index (KI) was used as an evaluation indicator. The study revealed that the gas injection rate shape significantly affected the mixture of hydrogen and air, thus impacting combustion, knock and emissions. When the injection rate shape was rectangle, the flame speed was faster, the peak pressure in the cylinder was higher, and the corresponding crank angle was earlier, which led to higher pressure oscillations in the cylinder and larger KI. Based on the rectangle injection rate shape, the KI decreased by 75.81%, 33.47%, 26.46% and 76.58% for trapezoid, wedge, slope, and triangle, respectively, and the indicated mean effective pressure increased by 15.68%, 5.07%, 0.56% and 14.98%, respectively. Due to the small difference in maximum temperature, which resulted in very little variation in nitrogen oxides for each injection rate shape, the total hydrocarbon emissions of the trapezoid and triangle injection rate shape was high due to the delayed combustion phase. This paper provides a solution for direct hydrogen injection to improve the combustion, knock and emissions behavior of the rotary engine.     

The effect of the initial charge temperature under various injection timings on the second law terms in a direct injection SI hydrogen engine

In this study, the effect of the initial charge temperature on the second law terms under the various injection timings in a direct injection spark ignition hydrogen fuelled engine has been performed theoretically during compression, combustion and expansion processes of the engine cycle. The first law analysis is done by using the results of a three dimensional CFD code. The results show a good agreement with the experimental data. Also for the second law analysis, a developed in house computational code is applied. The results reveal that the indicated work availability is more affected by varying hydrogen injection timing in comparison with other second law terms. Also increasing the initial charge temperature causes the heat loss availability and exhaust gas availability be increased and indicated work availability, combustion irreversibility and entropy generation be decreased.     

Research on the hydrogen injection strategy of a direct injection natural gas/hydrogen rotary engine considering apex seal leakage

The purpose of the present paper is to investigate the hydrogen injection strategy on the combustion performance of a natural gas/hydrogen rotary engine. Considering that apex seal leakage (ASL) is an inevitable problem in the actual working process of a rotary engine, the action of ASL cannot be ignored for an in-depth study of its combustion performance. Therefore, in this paper, a 3D dynamic simulation model that put the effect of ASL into consideration was established. Furthermore, based on the established 3D model, the combustion process of a natural gas/hydrogen rotary engine under various hydrogen injection angle (HIA) and hydrogen injection timing (HIT) was investigated. The results indicated that the hydrogen jet flow first impacted on the rotor wall after entering the cylinder, and then diffused under the action of the vortexes in the cylinder. Therefore, the HIA and HIT could change the hydrogen distribution by changing the hydrogen impact location and the intensities of the vortexes in the cylinder. In addition, the ideal hydrogen distribution at the ignition timing which could improve the combustion efficiency was given. That is, under the premise of ensuring minimized hydrogen leakage, the hydrogen should mainly distribute in the middle and the front of the cylinder, and a high hydrogen concentration is maintained near the spark plug.     

Numerical investigation of the effect of injection timing under various equivalence ratios on energy and exergy terms in a direct injection SI hydrogen fueled engine

In the present article, a Computational Fluid Dynamics (CFD) method and a home-made FORTRAN code have been utilized to investigate the effects of injection timing under various equivalence ratios on the first and second laws of thermodynamics terms in a hydrogen fueled Direct Injection Spark Ignition (DISI) engine. The results show a good agreement with the experimental data. Exergy terms such as exergy transfer with work, exergy transfer with heat, exergy transfer with exhaust gas and fuel chemical exergy were computed based on principles of the second law. Also Entropy generation per cycle is calculated. Special attentions are given to recognize and quantify the irreversibility of combustion process basing on the different injection timings and equivalence ratios. The obtained results indicated that combustion irreversibilities and exhaust gas availability are more affected by varying the equivalence ratio and amount of fuel availability that transfers to environment with exhaust gases increased by increasing the equivalence ratio. Varying the equivalence ratio had different effect on the accumulated work availability reduced to fuel availability at the late and early injection strategies. Also, entropy generation reduced by retarding the hydrogen injection timing and decreasing the equivalence ratio. Changing in injection timing has its maximum effect on Φ = 0.6 equivalence ratio.     

Numerical investigation of a large bore,direct injection,spark ignition,hydrogen-fuelled engine

This paper presents Unsteady Reynolds Averaged Navier Stokes (URANS) simulations of a large bore, hydrogen-fuelled direct injection spark ignition (DISI) engine at different spark and start of injection (SOI) timings. Six cases are simulated, including three with various spark timings at a low boost level and three with advanced to late injection timings at a higher boost level. The numerical simulations are validated with experimental data for four out of six cases, while the other two are considered blind computational fluid dynamics (CFD) simulations. It is shown that the autoignition occurs with advanced spark timing due to high in-cylinder pressure and unburnt temperature. For different SOIs, it is demonstrated that flame propagation involves a spark-initiated flame combined with an autoignition generated flame. The case with the late injection timing features poor mixing and slower combustion due to the presence of lean mixtures near the spark plug. As a result, this case features the lowest thermal efficiency when SOI is varied. In all cases, both mixture and temperature stratification are present. Simulations of zero-dimensional chemical reactors demonstrate that this stratification must be correctly captured for accurate prediction of autoignition timing.     

Numerical study on mixture formation characteristics in a direct-injection hydrogen engine

Numerical modeling of direct hydrogen injection and in-cylinder mixture formation is performed in this paper. Numerical studies on direct-injection hydrogen engines are very limited due mainly to the complexity in modeling the physical phenomena associated with the high-velocity gas jet. The high injection pressure will result in a choked flow and develop an underexpanded jet at the nozzle exit, which consists of oblique and normal shock waves. A robust numerical model and a very fine computational mesh are required to model these phenomena. However, a very fine mesh may not be feasible in the practical engine application. Therefore, in this study a gas jet injection model is implemented into a multidimensional engine simulation code to simulate the hydrogen injection process, starting from the downstream of the nozzle. The fuel jet is modeled on a coarse mesh using an adaptive mesh refinement algorithm in order to accurately capture the gas jet structure. The model is validated using experimental and theoretical results on the penetrations of single and multiple jets. The model is able to successfully predict the gas jet penetration and structure using a coarse mesh with reasonable computer time. The model is further applied to simulate a direct-injection hydrogen engine to study the effects of injection parameters on the in-cylinder mixture characteristics. The effects of the start of fuel injection, orientation of the jets, and the injector location on the mixture quality are determined. Results show that the hydrogen jets impinge on the walls soon after injection due to the high velocity of the gas jet. The mixing of hydrogen and air takes place mainly after wall impingement. The optimal injection parameters are selected based on the homogeneity of the in-cylinder mixture. It is found that early injection can result in more homogeneous mixture at the time of ignition. Results also indicate that it is more favorable to position the injector near the intake valve to take advantage of the interaction of hydrogen jets and the intake flow to create a more homogeneous mixture.     

Numerical simulation of the mixture distribution and its influence on the performance of a hydrogen direct injection engine under an ultra-lean mixture condition

In this study, a three-dimensional numerical model of a hydrogen direct-injection engine was established, and the combustion model was verified by experimental data. The influence of the injection timing and nozzle diameter on ultra-lean combustion was evaluated. The results suggest that, with the delay in the injection timing, the mixture concentration near the spark plug and combustion speed gradually increase. The maximum thermal efficiency increased from 47.44% to 49.87%. The combustion duration and ignition lag are shortened from 19.15°CA to 11.15°CA to 16.13°CA and 5.92°CA, respectively. As the nozzle diameter increased, the injection duration was shortened, and the mixture distribution area became more concentrated. Furthermore, under ultra-lean combustion, the combustion rate is more sensitive to the distribution of the mixture. Appropriately increasing the equivalence ratio near the spark plug can significantly shorten the ignition lag and combustion duration and obtain a higher thermal efficiency.     

Effects of split injection proportion and the second injection timings on the combustion and emissions of a dual fuel SI engine with split hydrogen direct injection

In this paper, a new kind of injection mode, split hydrogen direct injection, was presented for a dual fuel SI engine. Six different first injection proportions (IP1) and five different second injection timings were applied at the condition of excess air ratio of 1, first injection timing of 300°CA BTDC, low speed, low load conditions and the Minimum spark advance for Best Torque (MBT) on a dual fuel SI engine with hydrogen direct injection (HDI) plus port fuel injection (PFI). The result showed that, split hydrogen direct injection can achieve a higher brake thermal efficiency and lower emissions compared with single HDI. In comparison with single HDI, the split hydrogen direct injection can form a controlled stratified condition of hydrogen which could make the combustion more complete and faster. By adding an early spray to form a more homogeneous mixture, the split hydrogen direct injection not only can help to form a flame kernel to make the combustion stable, but also can speed up the combustion rate through the whole combustion process, which can improve the brake thermal efficiency. By split hydrogen direct injection, the torque reaches the highest when the first injection proportion is 33%, which improves by 1.13% on average than that of single HDI. With the delay of second injection timing, the torque increases first and then decreases. With the increase of first injection proportion, the best second injection timing is advanced. Furthermore, by forming a more homogeneous mixture, the split hydrogen direct injection can reduce the quenching distance to reduce the HC emission and reduce the maximum temperature to reduce the NO_X. The split hydrogen direct injection can reduce the HC emission by 35.8%, the NO_X emissions by 7.3% than that of single HDI.